Method of optical proximity correction with sub-resolution assists

ABSTRACT

A first aspect of the present invention is a method of determining an optical proximity correction for a primary feature having sub-resolution assist features for increasing the depth of focus of the primary features, comprising: generating a line/space pair; placing sub-resolution assist features on opposite sides of the line of the line/space pair; generating a set of linewidth biases; applying the set of linewidth biases to the line of the line/space pair to generate a set of biased-line/space pairs; determining for each biased-line/space pair, a deviation from a design linewidth of the line/space pair when the set of biased-line/space pairs are printed or simulated; and determining from the deviation a correction bias to apply to the line of the line/space pair. The invention also encompasses apparatus and computer programs for carrying out the methods.

BACKGROUND OF INVENTION FIELD OF THE INVENTION

[0001] The present invention relates to the field of opticallithography; more specifically, it relates to a method for correctingline width deviations.

[0002] Fabrication of modern integrated circuits typically involveslithographic transfer of a pattern disposed on a mask onto to a layer ofphotoresist on a substrate. The pattern on the mask defines theintegrated circuit patterns. It has been observed, especially as patternsizes have decreased, that differences in similar patterns in theintegrated circuit arise based on the proximity of patterns relative toone another. Therefore, various techniques for optical proximitycorrection (OPC) have been developed. It has also been observed, againas pattern sizes have decreased, that isolated and nested images focusdifferently in photolithographic exposure systems. Techniques, distinctfrom OPC, have been developed to address this problem. However, aspattern sizes continue to decrease well into the sub-micron region,linewidth control and image quality issues still continue to be aconcern and improved methods for linewidth control and image quality arerequired.

SUMMARY OF INVENTION

[0003] A first aspect of the present invention is a method ofdetermining an optical proximity correction for a primary feature havingsub-resolution assist features for increasing the depth of focus of theprimary features, comprising: generating a line/space pair; placingsub-resolution assist features on opposite sides of the line of theline/space pair; generating a set of linewidth biases; applying the setof linewidth biases to the line of the line/space pair to generate a setof biased-line/space pairs; determining for each biased-line/space pair,a deviation from a design linewidth of the line/space pair when the setof biased-line/space pairs are printed or simulated; and determiningfrom the deviation a correction bias to apply to the line of theline/space pair.

[0004] A second aspect of the present invention is a method ofdetermining a set of optical proximity correction rules for primaryfeatures having sub-resolution assist features for increasing the depthof focus of the primary features, comprising: generating a grating, thegrating comprising sets of sets of line/space pairs, each set of linespace/pairs comprising multiple copies of a unique combination of alinewidth value and a spacewidth value; generating a set of linewidthbiases; for each line/space pair of a particular set of line/widthpairs; selecting a sub-resolution assist features from a set ofsub-resolution assist features based on the spacewidth value of theline/space pair of the particular set of line/space pairs; placing thesub-resolution assist features on either side of each line of eachline/space pair of the particular set of line/space pairs; and applyinga different linewidth bias of the set of linewidth bias to each line ofeach line/space pair of the particular set of line/space pairs;determining deviations from design linewidths of the sets of line/spacepairs produced by the applying the different linewidth bias of the setof linewidth bias to each line of each line/space pair of the particularset of line/space pair; and generating from the deviations from thedesign linewidths of the set of line/space pairs the set of opticalproximity correction rules.

[0005] A third aspect of the present invention is a method of opticalproximity correction of primary features of a design havingsub-resolution assist features for increasing the depth of focus of theprimary features during operation of an optical lithography system bywidth biasing a light blocking layer on an optical mask, comprising: (a)selecting a set of line/space pairs representative of each the feature;(b) for each line/space pair of the set of line/space pairs; (i)generating, an identical set of line/space pairs representative of thefeature; (ii) placing sub-resolution assist features on opposite sidesof each line of each line/space pair, selection of the sub-resolutionassist features based on linewidth and space values of the line/spacepair; (iii) generating a set of linewidth biases; (iv) applying the adifferent linewidth bias of the set of linewidth biases to the line ofeach line/space pair to generate a set of biased-line/space pairs; (v)determining for each biased-line/width pair, a deviation from a designlinewidth of the line/space pair when the set of biased-line/space pairsare printed or simulated; and (vi) determining from the deviations, aset of optical proximity correction rules corresponding to eachline/space pair, each rule a correction bias; (c) selecting a featurefrom the primary features; (d) determining a corresponding line/spacepair from the set of line/space pairs representative of the feature; (e)placing sub-resolution assist features on opposite sides of the feature,selection of the sub-resolution assist feature based on linewidth andspace values of the line/space pair representative of the feature; (f)selecting a correction bias from the set of optical proximity correctionrules, selection of the correction bias based on linewidth and spacevalues of the line/space pair representative of the feature; and (g)applying the correction bias to the feature.

[0006] The invention also encompasses apparatus, systems and softwarefor carrying out methods of the invention.

[0007] These and other aspects of the invention are described in furtherdetail below.

BRIEF DESCRIPTION OF DRAWINGS

[0008] The features of the invention are set forth in the appendedclaims. The invention itself, however, will be best understood byreference to the following detailed description of an illustrativeembodiment when read in conjunction with the accompanying drawings,wherein:

[0009]FIGS. 1A through 1H illustrate possible sub-resolution assistfeatures (SRAF) placements according to the present invention;

[0010]FIG. 2 is a plot of measured image size versus design spaceillustrating the problem solved by the present invention;

[0011]FIG. 3 is a flowchart illustrating a generic method fordetermining optical proximity correction (OPC) rules according to thepresent invention;

[0012]FIG. 4 is a table illustrating exemplary data generated by thepresent invention that is used to create exemplary OPC rules accordingto the present invention;

[0013]FIG. 5 is a plot of measured image size versus design spaceillustrating the improvement provided by the present invention;

[0014]FIG. 6 is a flowchart of a first embodiment of the presentinvention;

[0015]FIG. 7 is a flowchart of a second embodiment of the presentinvention;

[0016]FIG. 8 is a flowchart of a third embodiment of the presentinvention; and

[0017]FIG. 9 is a flowchart illustrating creation of an optical maskaccording to the present invention.

DETAILED DESCRIPTION

[0018] When the term line/space pair is used, it should be understoodthat the width of the line and the width of the space between the lineand an adjacent line is being described. For the purposes of the presentinvention, a feature is defined as a line, a space or a line/space pair.It should also be understood that lines and spaces may be interchangeddepending upon the polarity of the mask supporting the features.

[0019] For the purposes of the present invention, the term printed isdefined as an actual measurement of a structure on a wafer. Themeasurement can be performed at any point from formation of a latentimage in a photoresist layer to a point after all processing of thewafer is complete. A printed image can be measured optically, byelectron microscopy or by electrical measurement.

[0020] Optical proximity correction (OPC) rules are designed to optimizethe exposure dose (light energy) given the focal properties of anoptical lithography system in order to produce printed lines that matchthe design widths of the lines. A simple example of an OPC rule wouldstate for a given line/space combination (exposed on a given opticallithographic system) the line width on the optical mask should beincreased (or decreased) by a specified amount in order to print thedesigned line width in the photoresist layer.

[0021] Sub-resolution assist features (SRAF) are features, too narrow tobe resolved by the optical lithographic system, added on either side ofa line to improve the sharpness of a line, whether or not it matches thedesign size. The effect of SRAFs are to make semi-isolated and isolatedlines behave more like nested lines (lines in close proximity to oneanother) since nested lines resolve with better depth of focus (sharperimages) then isolated lines in a given optical photolithographic system.

[0022] When SRAFs are used, three variables must be controlled. Thefirst is the width of each SRAF. The second is the width of the spacebetween the SRAF and line, and between SRAFs. The third is the number ofSRAFs. At one limit, the distance between adjacent lines is too small toput in any SRAFs. At the other limit, a maximum of four SRAFs may be putin (more than four generally adds no improvement in image quality).

[0023]FIGS. 1A through 1H illustrate possible SRAF placements accordingto the present invention. In FIG. 1A, there are no SRAFs placed betweenprimary lines 100. Lines 100 have a width L and the space between lines100 has a width S. The values of L and S constitute a line/spacecombination. In FIG. 1B, there is one SRAF 105 centered between primarylines 100. SRAF 105 has a width A1. In FIG. 1C, there is one SRAF 105Acentered between primary lines 100. SRAF 105A has a width A2. SRAF 105Ais wider than SRAF 105 in FIG. 1B (i.e. A2 is greater than A1). In FIG.1D, there are two SRAFs 105 between primary lines 110. The spaces S1 andS2 are equal. In FIG. 1E, there are two SRAFs 105 between primary lines110. The space S2 is greater than the spaces S1. In FIG. 1F, there arethree SRAFs 105 between primary lines 110. In FIG. 1F, The SRAF 105 toSRAF 105 distance and the SRAF 105 to line 100 distance are equal. InFIG. 1G, there are four SRAFs 105 between primary lines 110. In FIG. 1G,SRAF 105 to SRAF 105 distances and the SRAF 105 to line 100 distancesare equal. In FIG. 1H, a single isolated line 100 is illustrated withtwo SRAFs 105 on either side of the line.

[0024] In FIGS. 1A through 1H L and S are illustrated as being the same.This is generally not the case. In practice the number, width andplacement of SRAFs 105 are rule based, the rules being a function of Land S combinations. L and S are illustrated as being the same because asimulated line/space grating used in development of the presentinvention was constructed in that manner.

[0025] A simulated set of gratings having linewidths (in nm) of 150,175, 200, 225, 250, 275, 300, 350, 400 and 450 in combination withspacewidths (in nm) of 150, 175, 200, 225, 250, 275, 300, 350, 400, 450,500, 550, 600, 650, 100, 750, 800, 900, 950, 1000, 1200, 1400, 1600,1800, 2000, 2500, 3000, 3500, 4000 and 4500 was generated. This set ofgratings is exemplary and was used to generate the data illustrated inFIGS. 2, 4, and 5 and described infra. Other linewidth/spacewidthdimensions and combination may be used. Since there are 10 linewidthsand 31 spacewidths, the grating had 310 line/space pairs. Each gratinggenerally included seven to nine lines (about 2× the optical radius ofthe of the lithographic system). SRAFs were added to each gratingaccording to a set of rules as illustrated in FIGS. 1A though 1G.Exposure was simulated using an optical lithography system with anumerical aperture (NA) of 0.6 at a wavelength of 248 nm with an annularillumination of 0.75 nm outer and 0.5 nm inner standard deviation. Thedose was set for the 150/150 line/space pair. No OPC has been performed.The results are presented in FIG. 2. FIG. 2 is a plot of measured imagesize versus design space illustrating the problem solved by the presentinvention. An aerial image simulator was used to simulate exposure ofthe simulated grating described supra, and to measure the resultantimage sizes. In FIG, 2, the measured linewidth versus design space foreach set of linewidths is plotted. Curve 120 represents the 31 150/150to 150/4500 line/space pair sets. Curve 715 represents the 31 175/150 to175/4500 line/space pair sets. Curve 120 represents the 31 200/150 to200/4500 line/space pair sets. Curve 125 represents the 31 225/150 to225/4500 line/space pair sets. Curve 730 represents the 31 250/150 to250/4500 line/space pair sets. Curve 135 represents the 31 275/150 to275/4500 line/space pair sets. Curve 140 represents the 31 300/150 to300/4500 line/space pair sets. Curve 145 represents the 31 350/150 to350/4500 line/space pair sets. Curve 150 represents the 31 400/150 to400/4500 line/space pair sets. Curve 155 represents the 31 450/150 to450/4500 line/space pair sets.

[0026] The x or space axis is divided into seven regions: 160, 165, 170,175, 780, 185 and 190. Each region 160, 165, 170, 175, 180, 185 and 190corresponds to a different SRAF rule corresponding to FIGS. 1A through1G respectively. It can be readily seen that curves 110, 115, 120, 125,130, 135, 140, 145, 150 and 155 are not flat and exhibit a deviationrange of 166 nm from design nominal. Application of rule based OPCresults in a similar set of curves to those in FIG. 2 with in deviationrange of 122 nm. from design nominal, a relatively minor improvement ofonly 44 nm.

[0027] The reason for the failure of rule based OPC to sufficientlycorrect SRAF designs is that OPC rules are pitch based and assumeidentical optical environments for all line/space pairs within a pitchset. A pitch set is comprised of all line/space pairs having the samesum of the value of the linewidth added to the spacewidth. However,adding SRAFs results in different line/space pairs having differentoptical environments. For example, the pitch set of 1000 nm includes thefollowing 4 line/space pairs: 750/250, 600/400, 500/500 and 250/750. The750/250-line/space pair may have no SRAF, the 600/400-line/space pairmay have one SRAF, the 500/500-line/space pair may have one wider SRAFand the 250/750-line/space pair may have two SRAFs. The presentinvention provides for OPC rules that take into account the presence ofSRAFs.

[0028]FIG. 3 is a flowchart illustrating a generic method fordetermining OPC rules according to the present invention. In step 200, aseries of line/space pair sets is generated (e.g. using a real orsimulated grating as described supra). There are L different linewidthsand S different spacewidths for a total of L times S line/space pairs inS line/space pair sets. Line space/pair sets are designated as groups ofline/space pairs having a common spacewidth. The line/space pairs coverthe full range of design options of an integrated circuit. Also in step200, a set of bias values to apply to each linewidth is generated. Thereare B different bias values. Bias values include negative, zero andpositive values. The B bias values selected cover the full range of OPCvalues that the design/optical system would require.

[0029] In step 205, counters S (for line/space set) and L (for linewidthwithin the line/space set) are initialized to 1. In step 270, the nextline/space set is selected (and S is incremented by 1). In step 215, thenext linewidth L in line/space set S is selected, after which the Lcounter is incremented by 1. In step 220, SRAFs are added to the currentline/space pair based on SRAF rules 225. SRAF rules 225 are line/spacebased as discussed supra. Also, in step 220, B copies of the line/spacepair with SRAFs (if any) are generated.

[0030] In step 230, a different bias (as generated in step 200) isapplied to each line/space copy. The bias is applied only to the lineand not to any SRAFs. The key feature of the sequence is SRAFs arealways applied to lines having zero applied bias before applying bias tothose lines. In step 235 it is determined if there is another line/spacepair (another line value) in the current line/space pair set. If thereis then the method loops to step 215, if not the method proceeds to step240 where the L counter is reset to 1. Next, in step 245, it isdetermined if all line/space sets have been processed. If not, themethod loops to step 210. If all line/space sets have been processedthan in step 250, the all the line/space pairs are printed either realor simulated (there are L×S×B line/space pairs), and the deviation ofeach lines linewidth in each line/space pair from the designed linewidthfor that line is determined. When this data is arranged in a matrixsorted vertically first by spacewidth and then by linewidth andhorizontally by bias, a table is produced from which OPC rules that arecorrected for SRAFs may be calculated. This may be more easily seen byreference to FIG. 4. The table illustrated in FIG. 4 was derived usingthe simulated grating describe supra and the method illustrated in FIG.3 and described supra. FIG. 4 is a table illustrating exemplary datagenerated by the present invention that is used to create exemplary OPCrules according to the present invention. In FIG. 4, matrix 255, the Scolumn indicates the design width of the space in the line/space pairand the L column indicates the design width of the line in theline/space pair. Since the first sort is by space, the matrix isarranged in sets of line/pairs 260 having a common design space width.Columns 265 are sorted from most negative bias to most positive biasapplied to the line of each row (see step 230 of FIG. 3). In the presentexample, the bias ranges from −70 nm to 70 nm in 10 nm increments. Theapplied bias is indicated in a header row 270. The value in each cell ofmatrix 255 (excluding the L and S columns and header row 270) is thedeviation from design of the printed line. The actual data in FIG. 4 wasgenerated by simulation using the simulated set of gratings and SRAFsdescribed supra. An x indicates that there was no solution to the aerialimage. The OPC rule for each line/space pair is determined from a x-yplot of applied bias (row 270) along the y-axis and the deviation oflinewidth from design for a given line/space pair along the x-axis. Thex-intercept of the resultant curve is the amount of OPC correction to beapplied to that line/space pair when SRAFs have been incorporated intothe design. Of course, actual plots need not be made, and any number oftypes of curve fitting algorithms may be used to find the most accuratex-intercept. In the present example, the OPC bias that should be appliedto a line described by line/space pair 150/300 would be −13 nm.

[0031]FIG. 5 is a plot of measured image size versus design spaceillustrating the improvement provided by the present invention. The sameaerial image simulator as used supra was used to simulate exposure ofthe simulated grating after processing the grating as illustrated inFIG. 3 and described supra. In FIG, 5, the measured line width versusdesign space for each set of linewidths is plotted. Curve 110Arepresents the 31 150/150 to 150/4500 line/space pair sets. Curve 115Arepresents the 31 175/150 to 175/4500 line/space pair sets. Curve 120Arepresents the 31 200/150 to 200/4500 line/space pair sets. Curve 125Arepresents the 31 225/150 to 225/4500 line/space pair sets. Curve 730Arepresents the 31 250/150 to 250/4500 line/space pair sets. Curve 135Arepresents the 31 275/150 to 275/4500 line/space pair sets. Curve 140Arepresents the 31 300/150 to 300/4500 line/space pair sets. Curve 145Arepresents the 31 350/150 to 350/4500 line/space pair sets. Curve 150Arepresents the 31 400/150 to 400/4500 line/space pair sets. Curve 155Arepresents the 31 450/150 to 450/4500 line/space pair sets.

[0032] It can be readily seen that curves 110A, 115A, 120A, 125A, 130A,135A, 140A, 145A, 150A and 155A are very flat and exhibit a very smalldeviation range from design nominal of less than 4 nm.

[0033]FIG. 6 is a flowchart of a first embodiment of the presentinvention. In the first embodiment of the present invention, all OPCbiases are determined using simulation only. In step 275, a designgrating is created having L times S line/space pairs. The line/spacepairs cover the full range of design options of an integrated circuit.In step 280, SRAFs are added to the current line/space pair based onSRAF rules 225. In step 285, in simulation copies of the line/space pairwith SRAFs (if any) are generated. Also, in step 285, in simulation adifferent bias is applied to the line in each copy of each line/spacepair. The biases include negative, zero and positive values. The biasvalues selected cover the full range of OPC values that thedesign/optical system would require. Steps 275, 280 and 285 are similarto steps 200, 205, 210, 215, 220, 230, 235, 240 and 245 of FIG. 3.

[0034] In step 290 the negative, zero and positive bias cases (that isall cases) are simulated and the simulated deviation of each line widthof each copy of each line/space pair from design is determined. In step295, an OPC bias rule for each line/space pair is determined asdescribed supra in reference to FIG. 4, that is, by determining thex-intercept of the deviation of each simulated line width of each copyof each line/space pair from design versus applied bias. FIG. 7 is aflowchart of a second embodiment of the present invention. In the secondembodiment of the present invention, all OPC biases are determined usinga combination of simulation and actual printing and measurement.Essentially steps 290 and 295 of FIG. 6 are replaced with steps 300,305, 310, 315 and 320 in FIG. 7. Since steps, 275, 280 and 285 arerepeated in FIG. 7, they will not be discussed further.

[0035] After step 285, step 300 is performed. In step 300, a zero biasgrating is fabricated and printed. (The copies are made and multiplebiases are still applied in step 285, but that is in simulation). Instep 305, the printed lines are measured. In step 310, the negative,zero and positive bias cases (that is all cases) are simulated and thedeviation of each simulated line width of each copy of each line/spacepair from design is determined. Then in step 315, an offset between theprinted zero bias line widths and the simulated zero bias line widths isdetermined. This offset is applied to each simulated bias for allline/space pairs. In step 320, an OPC bias rule for each line/space pairis determined as described supra in reference to FIG. 4, that is, bydetermining the x-intercept of the deviation of each simulated linewidth (after applying the offset correction) of each copy of eachline/space pair from design versus applied bias. Steps 300, 305, 310 and315 in effect correct the simulation without requiring the more complexmask required by the third embodiment.

[0036]FIG. 8 is a flowchart of a third embodiment of the presentinvention. In the third embodiment of the present invention, all OPCbiases are determined using only actual printing and measurement.Essentially, steps 290 and 295 of FIG. 6 is replaced by steps 320 and325 in FIG. 8. Since steps, 275, 280 and 285 are repeated in FIG. 8,they will not be discussed further.

[0037] After step 285, step 325 is performed. In step 325, a mask havinggrating sets for the negative, zero and positive bias cases (that is allcases) is fabricated and the printed deviation of each line width ofeach copy of each line/space pair from design is determined. In step330, an OPC bias rule for each line/space pair is determined asdescribed supra in reference to FIG. 4, that is, by determining thex-intercept of the deviation of each printed line width of each copy ofeach line/space pair from design versus applied bias. FIG. 9 is aflowchart illustrating creation of an optical mask according to thepresent invention. FIG. 9 illustrates fabrication of an opaque layer ona glass mask according to the present invention wherein the lines ofeach line/space pair are fabricated in chrome. Opaque layers may befabricated from, for example, chrome, or light blocking material. Lightattenuating materials may be substituted for opaque material andcombinations of layers may be used. In step 350, a circuit design isselected. The circuit design determines the range of line/width pairsand OPC bias that will be required. In step 355, an exposure system andexposure parameters are determined. A simulated or actual grating havingSRAFs (from SRAF rules 225) is generated, a negative, zero and positivebias versus linewidth deviation table is generated and line/space pairbased OPC rules 360 are generated. Step 355 may employ any of the threeembodiments of the present invention as illustrated in FIGS. 6, 7 and 8and described supra. In step 365, a line (design feature) is selectedfrom the circuit design selected in step 350. In step 370, theline/space pair is determined for the current line. In step 375 theSRAFs (if any) are selected from SRAF rules 225 and applied to the line.In step 380 the OPC bias to apply to the line is selected from OPCline/space pair rules 360 and in step 385, the OPC is applied to theline. Steps 365, 370, 375, 380 and 385 are repeated until all lines inthe circuit design are corrected.

[0038] The data generated/used in the methods of the invention arepreferably embodied/stored in a computer and/or computer-readablemedium, and the steps of the invention are preferably performed using acomputer.

[0039] The invention also encompasses systems and/or apparatus forcarrying out the various method(s) of the invention. For example, theinvention encompasses systems and/or apparatus for determining anoptical proximity correction for a primary feature having sub-resolutionassist features for increasing the depth of focus of the primaryfeatures, comprising:(a)means for generating data describing aline/space pair;(b) means for generating data describing placement ofsub-resolution assist features on opposite sides of the line of theline/space pair;(c)means for generating data describing a set oflinewidth biases;(d)means for generating data describing a set ofbiased-line/space pairs by applying the data describing a set oflinewidth biases to a portion of the data describing a line/space pairthat corresponds to the line of the line/space pair;(e)means forgenerating data describing for each biased-line/space pair, a deviationfrom a design linewidth of the line/space pair when the set ofbiased-line/space pairs are printed or simulated; and(f)means forgenerating data describing a correction bias to apply to the line of theline/space pair based on the data describing the deviation.

[0040] Means (a)-(f) preferably comprises executable code stored in acomputer readable medium and a computer capable of executing the code.The system may further include a means for input/output of data and/orinterfacing with other software/computers used for generation of masklayout data.

[0041] The invention also encompasses computer programs stored in acomputer-readable medium for carrying out the method(s) of theinvention. For example, the invention encompasses computer programshaving computer-executable code for:(a) generating data describing aline/space pair;(b)generating data describing placement ofsub-resolution assist features on opposite sides of the line of theline/space pair; generating data describing a set of linewidthbiases;(d)generating data describing a set of biased-line/space pairs byapplying the data describing a set of linewidth biases to a portion ofthe data describing a line/space pair that corresponds to the line ofthe line/space pair;(e)generating data describing for eachbiased-line/space pair, a deviation from a design linewidth of theline/space pair when the set of biased-line/space pairs are printed orsimulated; and(f)generating data describing a correction bias to applyto the line of the line/space pair based on the data describing thedeviation.

[0042] The invention also encompasses a computer program stored in acomputer-readable medium, the program performing a method of determininga set of optical proximity correction rules for primary features havingsub-resolution assist features for increasing the depth of focus of theprimary features, the program comprising computer-executable codefor:(a) generating data describing a grating, the grating comprisingsets of sets of line/space pairs, each set of line space/pairscomprising multiple copies of a unique combination of a linewidth valueand a spacewidth value; (a)generating data describing a set of linewidthbiases;(b)for data describing each line/space pair of a particular setof line/space pairs;(i)selecting data describing a sub-resolution assistfeatures from data describing a set of sub-resolution assist featuresbased on the spacewidth value of the line/width pair of the particularset of line/space pairs;(ii)generating data describing placement of theselected sub-resolution assist features on either side of each line ofeach line/space pair of the particular set of line/space pairs;and(iii)generating data describing a biased-line/space pair by applyingdata describing a different one of the set of linewidth biases to aportion of the data describing a line/space pair that corresponds to theline of the line/space pair;(c) generating data describing deviationsfrom design linewidths of the respective biased line/space pairs;and(d)generating data describing the set of optical proximity correctionrules from the deviations.

[0043] The invention also encompasses a computer program stored in acomputer-readable medium, the program performing a method of opticalproximity correction of primary features of a design havingsub-resolution assist features for increasing the depth of focus of theprimary features during operation of an optical lithography system bywidth biasing a light blocking layer on an optical mask, comprising:(a)selecting data describing a set of line/space pairs representative ofeach feature;(b) for each line/space pair of the set of line/spacepairs;(i) generating data describing an identical set of line/spacepairs representative of the feature;(ii) generating data describingplacement of sub-resolution assist features on opposite sides of eachline of each line/space pair, the selection of the sub-resolution assistfeatures being on linewidth and space values of the line/spacepair;(iii) generating data describing a set of linewidth biases;(iv)generating data describing a biased-line/space pair by applying data fora different linewidth bias of the set of linewidth biases to the line ofeach line/space pair to generate a set of biased-line/space pairs;(v)generating data describing, for each biased-line/width pair, a deviationfrom a design linewidth of the line/space pair when the set ofbiased-line/space pairs are printed or simulated; and (vi) generatingdata, from the deviations, describing the set of optical proximitycorrection rules corresponding to each line/space pair, each rulecomprising a correction bias;(c) selecting data describing a selectedprimary feature;(d) determining a corresponding line/space pair from theset of line/space pairs representative of the selected primaryfeature;(e) generating data describing placement of sub-resolutionassist features on opposite sides of the selected primary feature,selection of the sub-resolution assist feature being based on linewidthand space values of the line/space pair representative of thefeature;(f) selecting data describing a selected correction bias fromthe set of optical proximity correction rules, selection of thecorrection bias based on linewidth and space values of the line/spacepair representative of the feature; and(g) applying the data for theselected correction bias to the data describing the selected feature thecourse of use, the computer programs of the invention may be resident ina computer which is part of a tool for design/generation of mask layoutdata. Alternatively, the programs of the invention may be in some othercomputer-accessible form (e.g., on separate computer, on portablecomputer-readable media (e.g., magnetic disk, hard drive, compact disk,etc.).

[0044] The description of the embodiments of the present invention isgiven above for the understanding of the present invention. It will beunderstood that the invention is not limited to the particularembodiments described herein, but is capable of various modifications,rearrangements and substitutions as will now become apparent to thoseskilled in the art without departing from the scope of the invention.Therefore, it is intended that the following claims cover all suchmodifications and changes as fall within the true spirit and scope ofthe invention.

1. A method of determining an optical proximity correction for a primaryfeature having sub-resolution assist features for increasing the depthof focus of said primary features, comprising: generating a line/spacepair; placing sub-resolution assist features on opposite sides of theline of said line/space pair; generating a set of linewidth biases;applying said set of linewidth biases to the line of said line/spacepair to generate a set of biased-line/space pairs; determining for eachbiased-line/space pair, a deviation from a design linewidth of saidline/space pair when said set of biased-line/space pairs are printed orsimulated; and determining from said deviation a correction bias toapply to the line of said line/space pair.
 2. The method of claim 1,wherein the step of placing sub-resolution assist features next to saidline/space pair is performed before the step applying said set oflinewidth biases to the line of said line/space pair.
 3. The method ofclaim 1, wherein said set of linewidth biases includes zero bias andbiases of positive and negative multiples of a constant value.
 4. Themethod of claim 1, wherein the step of determining from said deviation acorrection bias to apply to the line of said line/space pair includesdetermining a correction bias that results in substantially zerodeviation.
 5. The method of claim 1, wherein the step of determiningfrom said deviation a correction bias includes generating a function ofsaid deviation versus said set of applied linewidth biases and solvingsaid function for the correction bias having zero deviation.
 6. Themethod of claim 1, further including applying said correction bias tothe line of said line/space pair.
 7. A method of determining a set ofoptical proximity correction rules for primary features havingsub-resolution assist features for increasing the depth of focus of saidprimary features, comprising: generating a grating, said gratingcomprising sets of sets of line/space pairs, each set of linespace/pairs comprising multiple copies of a unique combination of alinewidth value and a spacewidth value; generating a set of linewidthbiases; for each line/space pair of a particular set of line/spacepairs; selecting a sub-resolution assist features from a set ofsub-resolution assist features based on said spacewidth value of saidline/width pair of said particular set of line/space pairs; placing saidsub-resolution assist features on either side of each line of eachline/space pair of said particular set of line/space pairs; and applyinga different linewidth bias of said set of linewidth bias to each line ofeach line/space pair of said particular set of line/space pairs;determining deviations from design linewidths of said sets of line/spacepairs produced by said applying said different linewidth bias of saidset of linewidth bias to each line of each line/space pair of saidparticular set of line/space pair; and generating from said deviationsfrom said design linewidths of said set of line/space pairs said set ofoptical proximity correction rules.
 8. The method of claim 7, whereinthe step of determining deviations from design linewidths of said setsof line/space pairs includes measuring simulated linewidths of eachline/space pair of each set of line/space pairs after adding saidsub-resolution assist features and said application of said linewidthbiases.
 9. The method of claim 7, wherein the step of determiningdeviations from design linewidths of said sets of line/space pairsincludes measuring printed linewidths of each line/space pair of eachset of line/space pairs after adding said sub-resolution assist featuresand said application of said linewidth biases.
 10. The method of claim7, wherein the step of placing sub-resolution assist features next tosaid line/space pair is performed before the step applying saiddifferent linewidth bias to each line of said line/space pairs.
 11. Themethod of claim 7, wherein said set of linewidth biases includes zerobias and biases of positive and negative multiples of a constant value.12. The method of claim 11 wherein the step of determining deviationsfrom design linewidths of said sets of line/space pairs includes:measuring printed linewidths of each line/space pair of each set ofline/space pairs after adding said sub-resolution assist features andapplication of said zero bias to each line of said line/space pairs inorder to generate a printed zero bias measurement set; measuringsimulated linewidths of each line/space pair of each set of line/spacepairs after adding said sub-resolution assist features and saidapplication of said linewidth biases in order to generate a simulatedbias measurement set for each linewidth bias of said set of linewidthbiases; calculating a set of offsets as the difference between saidprinted zero bias measurement set and said simulated bias measurementsets; and adding said set of offset to said deviations.
 13. The methodof claim 7, wherein the step of determining from said deviations alinewidth bias to apply to said line/space pair includes determining alinewidth bias that results in substantially zero deviation.
 14. Themethod of claim 7, wherein the step of determining deviations fromdesign linewidths includes generating for each said line/space pair, afunction of said deviations versus said set of applied linewidth biasesand solving said function for a correction bias having zero deviationfor each said line/space pair.
 15. A method of optical proximitycorrection of primary features of a design having sub-resolution assistfeatures for increasing the depth of focus of said primary featuresduring operation of an optical lithography system by width biasing alight blocking layer on an optical mask, comprising: (a) selecting a setof line/space pairs representative of each said feature; (b) for eachline/space pair of said set of line/space pairs; (i) generating, anidentical set of line/space pairs representative of said feature; (ii)placing sub-resolution assist features on opposite sides of each line ofeach line/space pair, selection of said sub-resolution assist featuresbased on linewidth and space values of said line/space pair; (iii)generating a set of linewidth biases; (iv) applying said a differentlinewidth bias of said set of linewidth biases to the line of eachline/space pair to generate a set of biased-line/space pairs; (v)determining for each biased-line/width pair, a deviation from a designlinewidth of said line/space pair when said set of biased-line/spacepairs are printed or simulated; and (vi) determining from saiddeviations, a set of optical proximity correction rules corresponding toeach line/space pair, each rule comprising a correction bias; selectinga feature from said primary features; (d) determining a correspondingline/space pair from said set of line/space pairs representative of saidfeature; (e) placing sub-resolution assist features on opposite sides ofsaid feature, selection of said sub-resolution assist feature based onlinewidth and space values of said line/space pair representative ofsaid feature; (f) selecting a correction bias from said set of opticalproximity correction rules, selection of said correction bias based onlinewidth and space values of said line/space pair representative ofsaid feature; and (g) applying said correction bias to said feature. 16.The method of claim 15, wherein the step of placing sub-resolutionassist features next to said line/space pair is performed before thestep applying said different linewidth bias to said line/space pair. 17.The method of claim 15, wherein said set of linewidth biases includeszero bias and biases of positive and negative multiples of a constantincremental value.
 18. The method of claim 15, wherein the step ofdetermining from said deviations a linewidth bias to apply to saidline/space pair includes determining a linewidth bias that results insubstantially zero deviation.
 19. The method of claim 15, wherein thestep of determining from said deviations a linewidth bias includesgenerating a function of said deviations versus said set of appliedlinewidth biases and solving said function for the correction biashaving zero deviation.
 20. The method of claim 15, wherein said featureare lines including material selected from the group consisting ofchrome, light attenuating material, light blocking material andcombinations thereof.
 21. A computer program in a computer readablemedium, said program comprising computer-executable code for carryingout the steps of claim
 1. 22. A computer program in a computer readablemedium, said program comprising computer-executable code for carryingout the steps of claim
 7. 23. A computer program in a computer readablemedium, said program comprising computer-executable code for carryingout the steps of claim 15.